add announcement model
add announcements to bulletins
add announcements from api

Announcements can be added via
POST /groups/:group_id/bulletins/:bulletin_id/announcements.
add ability to insert announcement at position
add position to serialized announcement
add ability to move announcement to new position
add ability to update description of announcement
delete announcements via api
Merge remote-tracking branch 'origin/master' into announcements
Merge branch 'master' into announcements
add specs for announcement error handling

Announcement error handling is also DRYed up.
require authentication for announcement actions
test authenticated actions via shared example

DRY up some controller specs to test for 302 status code. Decided
against making verifications that models are not actually saved. The
fact that a 302 has been explicitly set should suffice for now.
Merge pull request #8 from openmcac/announcements

Announcements